1. Field of the Invention
This invention relates generally to a bolster plate to support a substrate subject to high clamping forces for IC components, and more specifically relates to a bolster plate to support a printed circuit board assembled with one or more land grid array (LGA) IC components.
2. Description of the Prior Art
In many data processing systems (e.g., computer systems, programmable electronic systems, telecommunication switching systems, control systems, and so forth) very large pin count electrical components (e.g., application specific integrated circuits and processor chips) are assembled on substrates (e.g., printed circuit boards, other flexible substrates, multi-chip modules, and equivalents). One type of packaging that is frequently used for a very large pin count electrical component is what is commonly known as a land grid array (LGA) component. Electrical connections between the LGA component pins and the corresponding conductive pads on the substrate are frequently achieved by compressing an elastomeric insulating material containing several perpendicular conductive channels (e.g., buttons or columns filled with conductive balls or conductive threads).
In order to achieve reliable electrical connection between the pins and the pads, such LGA components are clamped by bolts to the substrate with high perpendicular clamping forces (exceeding several hundred pounds or several hundred newtons of force). However, such large perpendicular forces can cause distortion in the normally flat substrate, and the lack of flatness can cause poor electrical contacts that produce a permanent or an intermittent failure in system operation. Therefore, a bolster plate is frequently attached under a LGA component clamped area of the substrate to support the substrate. The bolster plate provides extra rigidity to the substrate, and the bolster plate helps to maintain the flatness of the substrate under the LGA component.
FIG. 1 illustrates a conventional bolster plate 102 assembled under a substrate (e.g., a printed circuit board) 104, opposite to the attachment of a LGA component 106 by clamp 108 into a socket 110 on the top of the substrate 104. The conventional bolster plate 102 is designed to provide flatness and rigidity to the substrate 104, and provide a uniform load distribution across the contact region of the LGA component 106 when the clamp 108 is bolted to the substrate 104 by bolts 114 and springs 112.
Conventional bolster plates are typically fabricated from a thick, heavy metal plate coated with one or more insulating layers, or they are fabricated from an expensive thermo-set composite material (e.g., a graphite fiber epoxy composite). Frequently, such bolster plates are constrained in thickness and dimensions by the increasing packing densities of nearby substrates. Thick and heavy bolster plates may touch and damage adjacent components on adjacent substrates, and can be especially detrimental to system reliability when the system is subject to shock and vibration. However, thin and light bolster plates may distort and fail to adequately support the substrate when high clamping forces are used for high pin count LGA components. Since the pin counts for IC components are continually increasing, clamping forces for LGA components and the resulting substrate distortions will increase. Without an improved bolster plate to counter such distortions, there will be an increasing number of poor electrical contacts between LGA components and the substrates, resulting in increasing operational and reliability failures.
It would be desirable to provide an improved bolster plate that can supply the necessary mechanical support to a substrate, and maintain the desired flatness of the electrical contact area under an IC component. In addition, this improved bolster plate would preferably be formed from thinner material, due to the reduction in open space adjacent to the substrate in a computer system.
The present invention provides an improved bolster plate that can supply the necessary mechanical support to a substrate, and maintain the desired flatness of the electrical contact area under an IC component. In addition, this improved bolster plate can be formed from thinner material, compared to the material needed in a conventional bolster plate.
A first aspect of the invention is directed to a method to assemble a pre-curved bolster plate to one side of a substrate having a first side and a second side. The method includes attaching a component to an electrical contact area on the first side of the substrate; and attaching the pre-curved bolster plate to the second side of the substrate, wherein the pre-curved bolster plate is attached to the second side opposite the electrical contact area on the first side of the substrate.
A second aspect of the invention is directed to a method to fabricate a pre-curved bolster plate. The method includes selecting a set of physical dimensions of the pre-curved bolster plate; estimating an initial radius of curvature for the pre-curved bolster plate; modeling the pre-curved bolster plate after assembly on a substrate; estimating a more precise radius of curvature for the pre-curved bolster plate after modeling the pre-curved bolster plate after assembly on the substrate; cutting the pre-curved bolster plate according to the set of physical dimensions; and stamping the pre-curved bolster plate to achieve the more precise radius of curvature.
A third aspect of the invention is directed to an assembled substrate. The assembled substrate includes a substrate having a first and a second side, and an electrical contact area on the first side; an electrical component having a plurality of leads attached to the electrical contact area of the substrate; and a pre-curved bolster plate attached to the second side of said substrate opposite the electrical contact area of the substrate.
These and other objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.